The document Fengfeng Tao, et al., “Self-Oscillating Electronic Ballast with Dimming Control”, Power Electronics Specialists Conference, 2001, IEEE, USA, describes a self-oscillating half-bridge inverter. FIG. 2 in this document shows a known half-bridge inverter with a feedback transformer. The electronic switches in this document are in the form of MOSFETs.
The cited half-bridge inverter is widely used as a circuit arrangement for operation of lamps, since it requires only a small number of low-cost components, and operates reliably. One disadvantage of this circuit arrangement is due to the fact that the oscillation frequency of the half-bridge inverter is governed by the feedback device that is used and, initially, there is no capability to influence the oscillation frequency. This is the case in particular with half-bridge inverters using bipolar transistors, since bipolar transistors are driven with high currents, which are complex to influence.
A capability to adjust the oscillation frequency offers the following advantages:                the oscillation frequency of the half-bridge inverter can be used to control the lamp current or the lamp power.        the preheating of lamp filaments can be adjusted by means of the oscillation frequency.        the lamp ignition process can be controlled by resonant ignition with the aid of the oscillation frequency.        the oscillation frequency of the half-bridge inverter can be set as a function of a temperature, thus allowing the lamp power and hence the power losses to be reduced when the ambient temperature is high or the installation conditions are poor.        
The first item of the above list is covered by the cited document. It is proposed that the feedback transformer be followed by a drive transformer. The oscillation frequency is now influenced by feeding a constant current to the drive transformer. The current that is fed in is superimposed on the magnetization current of the drive transformer, thus allowing the oscillation frequency to be adjusted. This solution has the disadvantage that the oscillation frequency depends on the magnetic characteristics of the drive transformer. Since these can be reduced only inaccurately, this solution is not suitable for large-scale production. Furthermore, the proposed solution does not have a closed control loop for lamp parameters such as the lamp current and lamp power.
The documents U.S. Pat. No. 4,525,648 (De Bijl) and U.S. Pat. No. 6,346,779 (Aiello) also propose self-oscillating half-bridge inverters with the capability to influence the oscillation frequency. However, once again, these documents do not disclose any control of lamp parameters such as the lamp current or lamp power.